I personally think that 3|0 is the hardest pool, based on pure observation( watching top games and taking interest in people's profiles) majority of GMs IMs, title players play this particular time control
You can test this yourself. Play 100 games of something like 3|2 and see if your rating goes up.
Bullet is the way to go
There are ways to essentially cheat, but chess.com doesn't care as long as you're not cheating in games.
Stronger the pool of players, the more deflated the ratings are.
This is true if the pools are separated by different ratings. However, 3|0 and 5|0 are both blitz ratings, which means that players that plays both 3|0 and 5|0 will inflate deflated ratings and deflate inflated ratings. Of course the strong pool of players migth still be a little bit deflated, but as long as it is enough people that plays in both pools, then this should be small.
15+10 rapid pool is stronger then 10+0 rapid pool even though they are both rapid. I guess casual players just immediately click on 10+0 game (this is the default setup) while only more serious players play 15+10 games regularily.
Rapid ratings are inflated compared to blitz ones, especially at that level. Most people have a rapid rating which is couple of hundreds points higher then their blitz rating. Were you playing against 1300 rapid players in 10+0 time format or in 10+15 time format?
At my level (I am rated 1800 rapid) I play against stronger players in 15+10 then in 10+0. If you play a 1800 rapid rated player in 10+0 format he is probably a weaker opponent then a 1800 rapid rated player that you meet when you play in 15+10 format.
I prefer to play longer games OTB as I can see my opponent, I can smell what he's thinking and nothing beats that fear smell, which you can't acomplish online.
Online is just rubbish , young kids using their engines and abusing you over net, if someone really loves chess they play OTB that's my opinion anyway.